This invention relates generally to digital multiplexing techniques and, more particularly, to multiplexing as applied to the transmission of digital communication signals over parallel narrowband channels. Local area networks (LANs) use various physical media for signal transmission between network stations or nodes. A widely used standard for communication through optical fibers is known as FDDI (fiber distributed data interface). Optical fibers provide an efficient and reliable data communication medium, but suffer from a significant disadvantage in that they are relatively costly. Pending application Ser. No. 07/545,310, filed on Jun. 26, 1990, now U.S. Pat. No. 5,119,402, and entitled "Method and Apparatus for Transmitting Local Area Network Signals Over Unshielded Twisted Pairs," proposes a solution to this difficulty, using a combination of techniques to transmit signals in accordance with FDDI protocols over parallel narrowband channels in the form of twisted pairs of copper conductors. The present invention is concerned with specific techniques for multiplexing and demultiplexing FDDI-type signals for transmission over parallel narrowband channels.
The use of parallel data channels or data paths is a commonly used technique in various other contexts, to increase the total rate of data flow. The parallel data channels may be two physically separate channels, such as separate twisted pairs of conductors, or may employ two frequencies on a single transmission medium (frequency-division multiplexing), or may employ phase division, with or without carrier signals. There are two commonly used demultiplexing approaches in the prior art.
One approach is to demultiplex data into N channels by simply dividing a binary data stream in a bit-by-bit fashion among the N channels. A first bit goes to the first channel, the next bit to the second channel, and so forth. The principal difficulty with this approach is that there will be possible occurrences of long run lengths of all ones or all zeros in any one channel. In most digital communication coding schemes, transitions of data signals between the zero and one states are used to derive a clock signal at the receiving end of the transmission. When the number of transitions is reduced, deriving a data clock signal from the data itself is rendered more difficult and a separate clock signal may be needed. Long run lengths of an unvarying signal state also require that the receivers be able to receive signals with a direct-current (dc) component. Receivers that are dc-coupled are more sensitive to loss and environmental noise, and are in general more costly than receivers that are not dc-coupled. The net result is that transmission distances are limited if this approach is used. Furthermore, if a separate clock signal is provided, there may be a skew error between the data and clock channels, which further limits the achievable transmission distance.
Another approach to demultiplexing is to transmit separate data packets over the separate channels. In local area networks, messages are encoded into self-contained packets of data, each of which contains its own required addressing and error-checking information. In this approach to demultiplexing, an original data packet is divided into N parts, for transmission over N channels. Then each part is separately encoded to include all the attributes of a data packet, including starting and ending delimiting codes to define the packet boundaries, a packet checksum, destination and source addresses, and other fields that are required in accordance with the communications protocol that defines a data packet. The obvious drawback of this approach is that it requires a large system overhead to demultiplex and multiplex the data, and replication of all the physical channel functions in each of the narrowband channels. Further, the necessary processing in each channel introduces a significant transmission delay.
It will be appreciated from the foregoing that there is a need for a simpler, yet effective approach to demultiplexing and multiplexing data transmitted through multiple narrowband channels. The present invention satisfies this need.